EP1607773B1

EP1607773B1 - Polarization sensitive solid state image sensor

Info

Publication number: EP1607773B1
Application number: EP04253649A
Authority: EP
Inventors: Jeffrey Raynor
Original assignee: STMicroelectronics Research and Development Ltd
Current assignee: STMicroelectronics Research and Development Ltd
Priority date: 2004-06-18
Filing date: 2004-06-18
Publication date: 2009-03-11
Anticipated expiration: 2024-06-18
Also published as: DE602004019888D1; US7186968B2; EP1607773A1; US20050279921A1

Description

The present invention relates to solid state image sensors, and in particular to a polarization sensitive solid state image sensor, and method of forming the same.
Current polarization sensitive solid state image sensors comprise a separately formed polarizing material which is mounted in the optical stack.
However, the polarizing material is difficult to align accurately, and may also move during the sensor's operation, which can lead to derogation or loss of polarizing function.
Moreover, in a solid state image sensor manufacturing process, the step of aligning the polarizing material adds to time taken and costs incurred.
Furthermore, using existing technology, it is difficult or impractical to produce an image sensor with two or more regions which are sensitive to different polarizations of light, for example an image sensor that comprises a first horizontally polarised image sensor and a second, vertically polarised image sensor. At present, manufacture of such an image sensor would require the use of separate materials, and subsequent individual realignment on the sensor. It would be desirable to allow for the practical manufacture of several pixels, each with their own particular polarization sensitivity, on the same device.
Reference is made to Chen et al: "A novel device for detecting the polarization direction of linear polarized light using integrated subwavelength gratings and photodetectors", IEEE Photonics Technology Letters, IEEE Inc., New York, US, vol. 9, no. 9, September 1997 (199709), pages 1259-1261, XP000721228 ISSN: 1041-1135. This discloses a polarization detector which makes use of a diffraction grating on a photodetector. However, in this document the grating is formed on a piece of fused silica which is bonded to the photodetector. Chen et al also shows the use of three gratings arranged at different angles on a common photodetector, for use in measuring the polarization direction; this requires the bonding of three fused silica items to the photodetector substrate.
Doumuki T et al: "An aluminum-wire grid polarizer fabricated on a gallium-arsenide photodiode", Applied Physics Letters, American Institute of Physics, New York, US, vol.71, no. 5, 4 August 1997, pages 686-688, XP000699633 ISSN: 0003-6951 discloses a polarization-sensitive photodetector in which a wire grid polarizer is patterned directly on the surface of a Ga-As photodiode in the form of aluminum wires by means of electron-beam lithography and lift-off. Since the patterning must be effected onto Ga-As this process is somewhat complex, and Doumuki et al does not show or suggest a polarizer integrated with an image sensor.
According to a first aspect of the present invention, there is provided a polarization sensitive solid state image sensor comprising:

a photodetector having a light-receiving surface comprising a dielectric layer; and
a polarizing assembly integrated with the photodetector and formed directly on the dielectric layer.

The polarizing assembly can cover all or any portion of an image sensing surface of the image sensor. Preferably, the polarising assembly comprises two or more regions which among them have two or more different polarisations.
Preferably, the polarizing assembly comprises a plurality of parallel metal lines.
Preferably, the metal lines have a pitch of a value less than a wavelength in the spectral range of visible light. Most preferably, the pitch is very much greater than 30nm.
Preferably, the parallel lines (12) are spaced apart by a distance equal to their width.
Preferably, the photodetector comprises a charge amplifier.
Optionally, the photodetector comprises a light to frequency converter.
According to a second aspect of the present invention, there is provided a method of forming a polarization sensitive solid state image sensor comprising forming a photodetector having a light-receiving surface comprising a dielectric layer; and forming polarizing assembly directly on said dielectric layer, whereby the polarizing assembly is integrated with the photodetector.
The plurality of parallel metal lines are preferably formed by a lithographic or etching process.
From further aspects, the present invention provides for an integrated circuit, an optical mouse, a digital camera, and a mobile telephone incorporating a digital camera, each of which comprises a polarisation sensitive solid state image sensor according to the first aspect, which can be formed by a method according to the second aspect.
The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a plan view of an image sensor in accordance with a first embodiment of the invention;
Fig. 2 shows a cross-sectional view taken across the line A-A' in Fig. 1;
Fig. 3 shows a charge amplifier suitable for use with the present invention;
Fig. 4 shows the variation of an output voltage of the charge amplifier of Fig. 3 over time;
Fig. 5 shows a light to frequency conversion circuit suitable for use with the present invention; and
Fig. 6 shows exemplary waveforms generated by the light to frequency conversion circuit of Fig. 5.

The present invention provides for a polarization sensitive solid state image sensor comprising an integrated photodetector and polarizing assembly. An embodiment of such a sensor is illustrated in Figs. 1 and 2.
Advances in CMOS technology have resulted in smaller dimensions for transistor gates, for example, 0.35µm, 0.18µm, or 0.13µm. Along with this, the dimensions of metal interconnects have also reduced in size, for example, 0.5µm, 0.32µm, or 0.16µm.
The wavelength of light in air is between 0.45µm and 0.65µm. As the sizes of these metal structures is less than the wavelength of light, they are therefore suitable for diffractive-type optics.
The short distance (1-3µm, as seen in Fig. 2) between the metal layers and the surface of the silicon is too short to allow interference, thus preventing the use of diffractive optics. However, this is not an issue for polarisors.
A polarizing assembly 10 according to a first embodiment of the present invention is shown in Figs. 1 and 2. An array of parallel metal lines 12 is provided having a width 14 of 0.16µm, and are spaced apart by a spacing 16 of 0.16µm. A pitch of the grid is defined as the summed width of the width 14 of one metal line and the spacing 16 adjacent to that line. Thus, the pitch of the illustrated array is 0.32µm.
As seen in Fig. 2, the metal lines 12 are formed directly on a dielectric layer 18 of a silicon photodetector. Thus, the image sensor produced comprises an integrated photodetector and polarising assembly. In this context, "integrated" is taken to mean that the polarising assembly is immovably attached in a fixed spatial relationship to a photodetector. These metal patterns can be produced very accurately as part of a normal manufacturing process, and thus the cost of incorporating a polarisation assembly is kept very low.
As the width 14 of the metal lines 12 and the spacing 16 between adjacent lines is equal, the illustrated array will allow approximately 50% of incident radiation to pass therethrough. It will be appreciated that the pitch can be varied by choosing different values of one or both of the width 14 of the metal lines 12 or the spacing 16 of the metal lines 12, in order to allow different proportions of incident light to pass through the array, as required.
Components of incident radiation having an electric field parallel to the array structure, i.e. parallel to the longitudinal axis of the metal lines 12, will pass relatively freely through the array. However, components of incident radiation having an electric field orthogonal to the array structure will collapse because the conducting properties of the metal lines 12 cause the electric field to collapse. It will be appreciated that any metal can be chosen, as long as it has a conductivity suitable to act effectively in an array as a polariser of incident light.
The efficiency of the polarizing assembly 10 is optimal if the pitch of the array is less than one tenth of the wavelength of incident radiation. For visible light, an optimal pitch is then 460nm/(10N_si) = 31.5nm, where N_si is the refractive index of silicon, having a typical value of 1.45.
Current high-volume manufacturing techniques cannot yet produce a structure this fine. However, the inventors have found that, surprisingly, a significant polarizing effect occurs with structures produced using a pitch which is very much greater than this optimal pitch. Accordingly, a pitch of very much greater than 30nm can be used to provide a polarizing effect which is useful for production of a polarization sensitive image sensor. In this context, "very much greater" than a particular figure means that the pitch must be at least twice that particular figure. In the illustrated embodiment, the pitch is approximately ten times the particular figure, i.e. over 300nm. The minimum allowable pitch will be governed by the manufacturing process.
The polarising structure can formed on the surface of any type of silicon photodetector, but one of high sensitivity is preferred as the polarisor introduces a significant attenuation. For example, in the embodiment illustrated in Fig. 2, at least 50% of incident light is lost.
A technique which is particularly suitable for this method is a light to frequency (LTF) converter. Examples of LTF converters are product numbers TSL235, TSL245 and similar devices available from Texas Advanced Optoelectronic Solutions. These include a photodiode and a current to frequency converter integrated on the same CMOS IC.
LTF converters are particularly useful for the present application because they employ a charge sensing technique. This permits the use of large area photodetectors, which collect more photons but have a large capacitance. More conventional readout techniques (e.g. 3 transistor) use a voltage sensing technique, so the large capacitance of the photodetector (the capacitance is proportional to its area) effectively cancels out the advantage of the greater number of photons collected by the larger area.
Fig. 3 illustrates a charge sensing amplifier that uses an operation amplifier 20 with a feedback capacitor, Cfb 22.
The amplifier 20 has a very high input impedance and so no current flows into it. The output will change so that the inverting input remains at the same potential as the non-inverting input. In doing so, a current will flow through the feedback capacitor Cfb 22. This will be of exactly the same magnitude (but opposite sign) to the photocurrent Ipd. Vout = -Ipd x Tint/Cfb (equation 1), and so the output voltage is independent of the pixel's capacitance.
Because of this, the architecture is very suitable for large photodiodes. The disadvantage is that the charge amplifier 20 needs to be reset periodically as the output voltage will swing outside the operating range of the amplifier 20, as shown in Fig. 4.
This reset can be achieved as part of a system's AEC (automatic exposure control). A practical method for resetting the charge-amp's feedback capacitor 22 is shown in Fig. 5, the readout signals from which are illustrated in Fig. 6.
As shown in Equation 1, the slope of the charge-amplifier's 20 output is proportional to the light. Hence the frequency of the light to frequency conversion is also proportional to the light. Fout = Ipd/(2 x Cfb x (Vthreshold - Vrt)) (equation 2).
The above described architecture provides a number of advantages, as follows:

· output signal is independent of photodiode capacitance/size
· Feedback capacitance can be chosen or designed for application.
· System is auto-exposing.
· Output is digital and therefore immune to noise
· ADC is included in the architecture
· Digital signal is easy to measure over large dynamic range (e.g.120dB).

Additionally, the above concepts can be applied in a manufacturing process to form a polarising structure on selected portions of an image sensor. These portions may or may not cover the entire image sensing surface. Furthermore, the orientation of the lines that are formed can be predetermined and can be different for different portions of the image sensor, so that different polarisations can be detected.
This polarisation sensitive image sensor can be incorporated in a number of different products, which include but are not limited to, a chip or integrated circuit, an optical mouse, and a digital camera provided as a separate unit or as part of a mobile telephone or webcam.
It will be appreciated that standard techniques may be employed by the man skilled in the art in order to implement the invention in these and other ways.
Various improvements and modifications can be made to the above without departing from the scope of the invention. In particular, an image sensor made in accordance with the principles of the invention may be incorporated in a product together with a non-polarized detector to gather information about ambient light.

Claims

A polarization sensitive solid state image sensor comprising:
a photodetector having a light-receiving surface comprising a dielectric layer (18);

a polarizing assembly (10) integrated with the photodetector and formed directly on the dielectric layer (18).
The image sensor of claim 1, wherein the polarizing assembly (10) comprises two or more regions which among them have two or more different polarisations.
The image sensor of claim 1 or claim 2, wherein the polarizing assembly (10) comprises a plurality of parallel metal lines (12).
The image sensor of claim 3, wherein the metal lines (12) have a pitch of a value less than a wavelength in the spectral range of visible light.
The image sensor of claim 4, wherein the pitch is very much greater than 30nm.
The image sensor of any of claims 3 to 5, wherein the parallel lines (12) are spaced apart by a distance (16) equal to their width (14).
The image sensor of any preceding claim, wherein the photodetector comprises a charge amplifier (20).
The image sensor of claim 7, wherein the photodetector comprises a light to frequency converter.
An integrated circuit comprising the image sensor of any of claims 1 to 8.
An optical mouse comprising the image sensor of any of claims 1 to 8.
A digital camera comprising the image sensor of any of claims 1 to 8.
A mobile telephone comprising the digital camera of claim 11.
A method of forming a polarization sensitive solid state image sensor comprising forming a photodetector having a light-receiving surface comprising a dielectric layer (18); and forming polarizing assembly (10) directly on said dielectric layer, whereby the polarizing assembly (10) is integrated with the photodetector.
The method of claim 13, wherein the polarizing assembly (10) is formed as two or more regions which among them have two or more different polarisations.
The method of claims 13 or 14, in which the polarizing assembly (10) is formed as a plurality of parallel metal lines (12) by a lithographic or etching process.